An important consideration in the selection and design of antennas is the propagation pattern of the free-space propagating electromagnetic wave. In a typical application, a transmitting antenna will transmit a guided electromagnetic wave to and from another antenna located on a device. The receiving antenna can be located in any number of directions from the transmitting antenna. Consequently, it is essential that the antennas for such wireless communication devices have an electromagnetic propagation pattern that radiates in all directions.
Another important factor to be considered in designing antennas for wireless communication devices is bandwidth of the antennas. Antennas need to operate at the specific bandwidth of the wireless device. Accordingly, antennas for use on these types of wireless communication devices are designed to meet the appropriate bandwidth requirements, otherwise communication signals will be severely attenuated.
The demand for compact and inexpensive antennas has increased as wireless communication has become commonplace in a variety of applications. Personal wireless communication devices, for example, cellular phones and Personal Data Assistants (PDAs) have created an increased demand for compact antennas. The increase in satellite communication has also increased the demand for antennas that are compact and provide reliable transmission. In addition, the expansion of wireless local area networks at home and work has also necessitated the demand for antennas that are compact and inexpensive.
The growing demand for wireless communication links in the 5.150–5.875 GHz bandwidth range requires low cost omnidirectional radiators. Moreover, these radiators should exhibit wideband operation and high gain. The radiation pattern is required to be omnidirectional in the azimuth direction with small variation in the gain in all directions (typically less than 2 decibels (dB)).
The above requirements make the design of these radiators challenging. While series-fed collinear radiators provide enough gain and radiate in an omnidirectional pattern, they are inherently narrowband and the main lobe radiation beam is frequency dependent in the elevation plane.
One way to increase the bandwidth of antennas is to make a corporate network feeding multiple broadband radiating elements. The corporate network comprises the feed lines that supply the feed signal. Using multiple radiating elements have to overcome the problems associated with limited space within the antenna enclosure, along with placing the broadband radiating elements in a pattern to radiate in all directions.
Planar structures have been proposed to include corporate networks and the radiating elements on the same plane. This kind of construction has the advantage of low cost and manufacturing repeatability, but it comes with disadvantages. The number of feeding lines for the corporate network as well as radiating elements is limited by the width of the board supporting the antenna components. Slot radiators placed along the board, fed by microstrip feed lines, require a larger amount of space on the board and limit the number of microstrip feed lines for the corporate network. Moreover, the microstrip feed lines are located close to the slots, coupling unwanted electromagnetic energy. In addition, the radiation patterns produced by the slot radiators have a limited omnidirectional radiating pattern.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.